
The Harrison Instruments site ain't too much to look at, but they do have the biggest selection of Theremin kits I've ran across. A Theremin is one of the first electronic instruments, it uses radio frequencies to make noise as your hands get closer to it... Link.
Theremins galore!:

DIY RCA Theremin Replica - Link.

HOW TO - Make a Theremin - Link.

What is a Theremin? - Link.

HOW TO - Make a Radio Shack optical Theremin - Link.

Optical theremin + soapdish - Link.
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More kits from Jaycar Australia here:
http://www1.jaycar.com.au/productView.asp?ID=KC5295
http://www1.jaycar.com.au/productView.asp?ID=KC5426
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